Good instructable!I did a lot of time ago something similar, but I went a bit crazy and implemented all the functions using a single chip (a dsPIC). It does signal acquisition, FSK decoding, CID command parsing, audio playback using PWM technique, SD card handling (for storing a call log and for reading audio files), etc. If interested, you can find it here:http://kernelhacks.blogspot.com/2013/12/balsamo-re...There is also a github repo with schematics and sources:https://github.com/doragasu/Balsamo